1. Field of the invention
The present invention relates to a method for controlling multi-phase chemical reactions using the architecture of surfactant-based foams to control mass transport of chemical reactants, catalysts, and products and the kinetics with which they react. More specifically, the invention relates to transformations that require both gaseous and liquid components with dissolved or suspended reactants that are unstable when sheared.
2. Related Art
Chemical reactions that simultaneously involve gases, liquid solutions, and suspended solids are ubiquitous, particularly in biologically inspired chemistry. Fermenters, incubators, and culture bags have been widely used as bioreactors to grow and manipulate cells; these combine gases such as O2 or CO2, liquids such as water or aqueous solutions, and solids such as suspended proteins, liposomes, and vesicles to enable complex reactions such as photosynthesis or respiration.
A problem with these reactors is that the small surface area defined by the gas-liquid interface constrains the rate of gas transport into the liquid phase. A second problem with the prior art involves the long characteristic diffusion times of gases dissolved in the liquid phase. These problems are partially, not completely, addressed by stirring or by continuously circulating gas bubbles through the liquid. Either method increases the area of the gas-liquid interface and supplements diffusive with convective mixing of the reactants. However, stirring and bubbling generates fluid mechanical shear in the reactive mixture. As is known to those practiced in the art of biochemistry, enzymes are catalysts formed from proteins. The ability of these enzymes to catalyze reactions is crucially dependent on their three-dimensional conformation in solution, and this can be irreversibly altered by fluid dynamical shear of protein suspensions and solutions. For example, lyophilized pharmaceuticals often loose potency when shear produced by vigorous mixing is used. The role of shear in degradation of proteins is thoroughly reviewed by Thomas and Geer, Effects of Shear on Protein Solutions, Biotechnology Letters, 33, 443-56 (2011), which is expressly incorporated herein by reference. This rheological instability of the enzymes constrains optimal mixing of reactants and therefore reduces rates and efficiencies for production of desired compounds.
The prior art partially addresses the challenge of component destruction by shear. The component most sensitive to shear (cells), are mechanically filtered from the less sensitive component (broth) in their two chamber bioreactor. The broth is treated with gas by bubbling and stirring, and then remixed with the shear-sensitive component. This approach suffers from three limitations: (i) a two-part reactor and filter are required, (ii) any shear-sensitive component that is not completely separated during filtration will be sheared, (iii) reactions that proceed in the bubbling chamber must produce products that are stable long enough to be convectively pumped back into the main reactor.
Another problem with bubbling is that it often requires addition of surfactants that alter surface tension in the fluid to enable bubble formation. Traditional surfactants and detergents often interfere with lipid vesicles, membranes, and proteins, thereby reducing the rates and yields achievable in reactors where they are required. The International Published Application WO 2006/089245 is directed to a bubble architecture and method of making such a bubble, the contents of which are expressly incorporated herein by reference. Although this document lists a variety of surfactants that may be used in making a bubble or foam, the Examples are primarily directed to the use of TWEEN-20™, which exemplify the limitations caused by chemical interactions between surfactants and reactants.
Yet another problem with prior art involving bubbles concerns control of the bubble structure for times that are long compared to those required for the desired chemical transformations. When gas is bubbled through a long column, the lifetime of a bubble-liquid interface is governed by buoyancy, viscosity, and surface tension of the fluid as well as the geometry of the column. This lifetime is not explicitly coupled to the timescales required for chemical reaction, a limitation that is overcome in the method of the current invention.
Bubble architectures and methods of making and using such bubble architectures, wherein the bubble architectures are formed using biologically derived surfactant, for example, the protein Ranaspumin-2 and other biologically derived surfactants to create functional materials that mimic cellular physiological processes has been disclosed in International Patent Application PCT/US10/60610 filed on Dec. 15, 2010 claiming priority to U.S. Provisional application No. 61/286,578 filed Dec. 15, 2009, the entire contents of which are expressly incorporated herein by reference.
The foam nest produced by the Tungara frog is one of the largest found in nature. It is used to protect developing tadpoles in terrestrial areas of tropical and subtropical Central America, until maturation or greater water availability. The creation and maintenance of the Tungara frog's foam nest can be attributable to a suite of six proteins called ranaspumins (Rsn1-6). Of these, Rsn2 is responsible for the reduction in water surface tension allowing foam creation upon liquid agitations. The other ranaspumins resist microbial infection and insects, and provide carbohydrate binding to stabilize the foams to drainage and desiccation. Rsn-2 plays the surfactant role very economically at concentrations as low as 0.1 mg/ml, but also has the ability to exist in two conformational states (see Mackenzie, C. D., et al., Ranaspumin-2; Structure and Function of a Surfactant Protein from the Foam Nests of a Tropical Frog, Biophysical Journal, 2009, 96(12); p. 4984-4992, which is expressly incorporated herein by reference). When agitated, the protein denatures slightly, allowing the single hydrophobic alpha helix to extend into the air while the hydrophilic beta sheet remains in the water phase. Normally these two regions are folded onto each other, so without agitation or continued bridging of the air-water interface, the protein is most likely to exist as an invert water-soluble protein. The foam nests of the Tungara frog are one example of a protein based foam that is compatible with lipid membranes, yet resistant to environmental factors. Another example of a surfactant protein according to the present invention is Ranasmurfin, which is produced by a Java frog (Oke et al., Unusual Chromophore and Cross-Links in Ranasmurfin: A Blue Protein from the Foam Nests of a Tropical Frog, Angew. Chem. Int. Ed. 2008, 47, 7853-7856, which is expressly incorporated herein by reference). The persistence of these foams can be adjusted from minutes to more than three days, the time required for tadpole maturation, by changing concentrations and compositions of the ranaspumin or ranasmurfin proteins. (see Downie, J. R., Functions of the foam in foam-nesting Leptodactylids: the nest as a posthatching refuge in Physalaemus pustulosus. Herperol, J 1993. 3: p. 35-42.)